Databases
StackOverflow2013
Postsdbo
PODead lock without a explicit lock
 

Note that there are some explanatory texts on larger screens.

plurals
  1. PODead lock without a explicit lock
    primarykey
    data
    text
    <p>I am testing a pthread program.</p> <p>This program is simple. The main thread creates a child thread. </p> <p>The main thread and the child thread are both operating on a queue.</p> <p>The child thread keeps scanning the queue and return the minimal element and its position with a infinite loop.</p> <p>The main thread also is running a loop, each iteration of which delete the minimal element calculated by the child thread from the queue, and insert some new elements to the end of the queue.</p> <p>The minimal element and its position, and the queue are all declared as global variables.</p> <p>The main ends when the queue is empty and it will cancel the child thread.</p> <p>This progress is some like a breadth-first search.</p> <p>The queue is implemented as an array with a size counter. The deletion operation is implemented as replacing the element to be deleted by the last element and decreasing the size counter by one.</p> <p>No lock is used here. But when running, the program will get stuck.</p> <p>What's more amazing, if I insert some printf statements to view the status, it may finish.</p> <p><strong>I want to know what causes this program endless?</strong></p> <pre><code>struct multiblocks_pthread_args { volatile int local_e; volatile int local_v; volatile int local_pos; int* Q; int* val; volatile int* size; } para; volatile int update = 0; void* child_thread ( void* args ) { pthread_setcanceltype ( PTHREAD_CANCEL_ASYNCHRONOUS, NULL ); multiblocks_pthread_args* arglist = ( multiblocks_pthread_args* ) args; bindToCore ( 1 ); int* list = arglist -&gt; Q, * value = arglist -&gt; val; while ( true ) { int size, e, v, pos; do { size = * ( arglist-&gt;size ), e, v = INF, pos = 0; update = 0; for ( int i = 0; i &lt; size; i++ ) { int vi = value[i]; if ( vi &lt; v ) { pos = i; v = vi; } } } while ( update ); if ( size &gt; 0 ) e = list[pos]; arglist-&gt;local_e = e; arglist-&gt;local_pos = pos; arglist-&gt;local_v = v; } return NULL; } void main_thread () { int size; int* Q = ( int* ) malloc ( sizeof ( int ) * NumNode ); int** hash = ( int** ) malloc ( sizeof ( int* ) * numNode ); NodeColor* color = ( NodeColor* ) malloc ( sizeof ( NodeColor ) * numNode ); // NodeColor is a enum with 3 values: WHITE, GRAY, BLACK memset ( color, 0, sizeof ( NodeColor ) * numNode ); pthread_t tid; para.val = ( int* ) malloc ( sizeof ( int ) * NumNode ); para.Q = Q; para.size = &amp;size; pthread_create ( &amp;tid, NULL, child_thread, &amp;para ); // Only one element is in the queue size = 0; para.Q[size] = 0; para.val[size] = 0; hash[0] = &amp;para.val[size]; // hash is used to modify the value of particular element ++size; color[0] = GRAY; while ( true ) { int global_e, global_v = INF, global_pos; global_e = para.local_e, global_v = para.local_v, global_pos = para.local_pos; if ( size == 0 ) break; if ( color[global_e] != BLACK ) { value[global_e] = global_v, color[global_e] = BLACK; if ( size &gt; 0 ) { --size; para.Q[global_pos] = para.Q[size]; para.val[global_pos] = para.val[size]; hash[para.Q[global_pos]] = &amp; para.val[global_pos]; update = 1; } for ( int i = 0; i &lt; MAXDEG; ++i ) { int ee = ;// new element; int vv = ;// value of new element; if ( /* if new element is valid */ ) { if ( color[ee] == WHITE ) { // WHITE means ee is not in the queue para.Q[size] = ee; para.val[size] = vv; hash[ee] = &amp;para.val[size]; ++size, color[ee] = GRAY; } else { *hash[ee] = vv; } update = 1; } } } } free ( Q ); pthread_cancel ( tid ); printf ( "Computation finishes!!!" ); return ; } </code></pre>
    singulars
    1. PO
      singulars
      1. PTAnswer
    1. This table or related slice is empty.
    1. PTQuestion
    1. USkonjac
    1. USkonjac
    plurals
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. PO
      singulars
      1. PTAnswer
    1. This table or related slice is empty.
    1. COShared variables + no locks => data races, undefined behavior
      singulars
      1. PODead lock without a explicit lock
      1. USMat
    2. COIf you don't have explicit locks, you must have some spin waiting. Is it possible that there's some edge case where all the threads loop endlessly?
      singulars
      1. PODead lock without a explicit lock
      1. UShawk
    3. COOh, and printf might introduce some synchronization which might make your program "more correct". Without seeing your code, we can only assume you're doing something wrong. Can't really help.
      singulars
      1. PODead lock without a explicit lock
      1. USMat
 

Querying!

 
Guidance
A row detail

Detail views are divided into sections. All the information in the data section comes from columns in the selected row. The other sections display data from other, related rows.

Related data can be related in a to-one or a to-many fashion. Captions of data related in a to-many fashion link to a list view showing a filtered view of the table.

Try moving around until you find a non-empty to-many entry and click on the label to get to one. You can move back to the root by clicking on the database name in the header.

SQuiL has stopped working due to an internal error.

If you are curious you may find further information in the browser console, which is accessible through the devtools (F12).

Reload